GUI movement Control. Please Help!!! Thanks

{
public float speed = 0.00f;
public float rotSpeed = 10.0f;
public float strafeSpeed =10.0f;
private float engineSpeed = 0.0f;
public float pitchConstant = 1.0f;
public float yawConstant = 0.0f;
public GameObject boostersprefab;
public GameObject boosterPos;

	private GameObject instantiated;
    private bool instanBoostersExist;		
	
	// GUI texture
	
	public GUITexture forwardPitchButton;
	public GUITexture backwardPitchButton;
	public GUITexture leftPitchButton;
	public GUITexture rightPitchButton;		
	
	
	
	
	// Booster Start Function (Instantiate the particles)
	void boostersStart()		
	{  
        if(instanBoostersExist == false)	
		
		{			
		  instantiated = (GameObject)Instantiate(boostersprefab, boosterPos.transform.position, boosterPos.transform.rotation);
		  instantiated.transform.parent	= boosterPos.transform;
		  instanBoostersExist = true;			
		}		
	}		
	
	// Booster Stop Function (Stop Instantiating the particles)
	void boosterStop()
	{		
	    if (instanBoostersExist == true)
		{		
			Destroy(instantiated,0.2f);
		    instanBoostersExist = false;
		}	
	}
	
		
	
	void Update()
		
	{		
	// Axis Declaration	
	float pitch = Input.GetAxis("Pitch")*pitchConstant; 		
	float yaw   = Input.GetAxis  ("Yaw")*yawConstant;		
	float roll =  Input.GetAxis ("Roll");
	float gas =   Input.GetAxis ("Gas");		
	
		
	Vector3 strafe = new Vector3 (Input.GetAxis("Horizontal")*strafeSpeed*Time.deltaTime,Input.GetAxis("Vertical")*strafeSpeed*Time.deltaTime,0);// Vertical and Horizontal Axis is feeded to the strafe movement.		
			
		// Engine Mechanism		
		
		 if (engineSpeed <10 && engineSpeed > -3)
		 {
		  engineSpeed += gas;			
		  boostersStart();		  
		 }
		
		 if(engineSpeed > 10)
		 {
		  engineSpeed = (9.99f);	
		 }
		
		 if (engineSpeed<-3)
		 {
		  engineSpeed = (-2.99f);		
		 }
		
	      else if(Input.GetKey("z"))// Brake
		 {
		  engineSpeed = (0.00f);
		  boosterStop();		  
		 }	
	
	 
		
		
		
		
	// SpaceShip Physics Movement
		
	rigidbody.AddRelativeTorque(pitch*rotSpeed*Time.deltaTime,yaw*rotSpeed*Time.deltaTime,roll*rotSpeed*Time.deltaTime); // realative torque is added
	
	rigidbody.AddRelativeForce(0,0,engineSpeed*speed*Time.deltaTime); // relative force	
		
	rigidbody.AddRelativeForce(strafe);	// rigidbody added to the strafe movement
		
		
	}	
}

How do I feed the movement function to GUI button?, I already created GUI texture and declared it int he program. I tried many way but couldn’t get it working. i just want to move the ship when the GUI button is pressed instead of keyboard controls. Please help me out. Thanks a lot.

Change this

 else if(Input.GetKey("z"))// Brake
{
engineSpeed = (0.00f);
boosterStop();
}

to something like

if(isBraking) {
engineSpeed = (0.00f);
boosterStop();
}

in your OnGUI you’ll have button named ‘Brake’

if(GUI.Button,.....,) isBraking=true;